Window watchdog concept in Embedded C - Step-by-Step Execution

Concept Flow - Window watchdog concept

Start Program

↓

Initialize Window Watchdog

↓

Enter Main Loop

↓

Check if Refresh Window Open?

No→System Reset

Yes↓

Refresh Watchdog Counter

↓

Continue Normal Operation

↩Back to Main Loop

The program initializes the watchdog, then repeatedly checks if the refresh happens within the allowed time window. If refreshed too early or too late, the system resets.

Execution Sample

Embedded C

WWDG_Init();
while(1) {
  if (within_refresh_window()) {
    WWDG_Refresh();
  } else {
    // do not refresh
  }
}

This code initializes the window watchdog and refreshes it only when inside the allowed time window to prevent system reset.

Execution Table

Step	Watchdog Counter	Time	Condition: Inside Window?	Action	System State
1	Start at max (e.g. 127)	0 ms	N/A	Initialize watchdog	Running
2	126	10 ms	No (too early)	No refresh	Running
3	125	20 ms	Yes (inside window)	Refresh watchdog	Running
4	127	21 ms	No (too early after refresh)	No refresh	Running
5	125	30 ms	Yes (inside window)	Refresh watchdog	Running
6	127	31 ms	No (too early)	No refresh	Running
7	126	40 ms	No (too late, missed window)	No refresh	System Reset
Exit	-	-	-	-	System reset triggered due to missed refresh window

💡 System resets because the watchdog was not refreshed within the allowed time window.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	Final
Watchdog Counter	127	126	127	126	127	126	126	-
System State	Running	Running	Running	Running	Running	Running	System Reset	System Reset

Key Moments - 2 Insights

Why can't we refresh the watchdog counter too early?

Refreshing too early means outside the allowed window, so the watchdog treats it as a fault and will reset the system, as shown in steps 2 and 4 where no refresh happens because it's too early.

What happens if we miss the refresh window completely?

If the refresh window is missed, the watchdog counter reaches a critical value and triggers a system reset, as seen in step 7 where the system resets due to no refresh.

Visual Quiz - 3 Questions

Look at the execution table, what is the watchdog counter value at step 3 after refresh?

A 126

B 125

C 127

D 124

💡 Hint

Step 3 shows 125 before refresh, but after refresh reloads to max 127 (as step 4 starts at 127).

At which step does the system reset occur according to the execution table?

A Step 6

B Step 7

C Step 5

D Step 4

💡 Hint

Look at the 'System State' column for the first occurrence of 'System Reset'.

If the watchdog is refreshed too early at step 2, what would happen?

A System reset triggered immediately

B Watchdog counter increases

C System continues running normally

D Watchdog counter freezes

💡 Hint

Refer to the explanation in key_moments about refreshing too early causing reset.

Concept Snapshot

Window Watchdog Concept:
- Initializes with a counter that counts down.
- Must refresh only within a specific time window.
- Refreshing too early or too late causes system reset.
- Helps detect software faults by enforcing timing constraints.
- Used in embedded systems for safety and reliability.

Full Transcript

This visual execution trace shows how a window watchdog timer works in embedded C. The watchdog counter starts at a maximum value and counts down over time. The program must refresh the watchdog only when inside a specific time window. Refreshing too early or too late causes the watchdog to reset the system. The execution table shows each step with the watchdog counter value, time, condition check, action taken, and system state. Variable tracking shows how the watchdog counter and system state change over time. Key moments clarify why refreshing too early or missing the window causes reset. The quiz questions test understanding of the watchdog counter values and reset timing. This concept helps embedded programmers ensure their system recovers from software faults by enforcing timely refreshes.

Practice

1. What is the main purpose of a window watchdog timer in embedded systems?

easy

A. To increase the processing speed of the microcontroller

B. To reset the system if the software fails to refresh the timer within a specific time window

C. To store data temporarily during program execution

D. To provide power backup during system failure

Solution

1. Step 1: Understand the watchdog timer function

   A watchdog timer monitors if the software is running correctly by expecting periodic refreshes.

2. Step 2: Identify the role of the window watchdog

   The window watchdog resets the system if the refresh happens too early or too late, ensuring software reliability.

3. Final Answer:

   To reset the system if the software fails to refresh the timer within a specific time window -> Option B

4. Quick Check:

   Window watchdog resets system on missed or mistimed refresh [OK]

Hint: Watchdog resets system if refresh is outside allowed time [OK]

Common Mistakes:

• Confusing watchdog with memory storage
• Thinking watchdog speeds up the processor
• Assuming watchdog provides power backup

2. Which of the following is the correct way to refresh a window watchdog timer in embedded C?

easy

A. Call WWDG_Refresh() function within the allowed time window

B. Call WWDG_Refresh() only once at system startup

C. Disable the watchdog timer before refreshing

D. Refresh the watchdog timer continuously without delay

Solution

1. Step 1: Understand watchdog refresh requirements

   The watchdog must be refreshed periodically within a specific time window to avoid reset.

2. Step 2: Identify correct refresh method

   Calling WWDG_Refresh() within the allowed time window is the correct way to refresh.

3. Final Answer:

   Call WWDG_Refresh() function within the allowed time window -> Option A

4. Quick Check:

   Refresh watchdog periodically inside time window [OK]

Hint: Refresh watchdog periodically, not just once or too fast [OK]

Common Mistakes:

• Refreshing only once at startup
• Disabling watchdog before refresh
• Refreshing too frequently causing reset

3. Consider this code snippet using a window watchdog timer:

void main() {
  WWDG_Init();
  while(1) {
    delay(50);  // delay in ms
    WWDG_Refresh();
  }
}

If the allowed refresh window is between 40ms and 60ms, what will happen?

medium

A. The system will reset because refresh is too early

B. The system will reset because refresh is too late

C. The system will run normally without reset

D. The system will freeze due to watchdog error

Solution

1. Step 1: Analyze the refresh timing

   The code delays 50ms before refreshing the watchdog, which is within the 40-60ms window.

2. Step 2: Determine system behavior

   Since refresh happens inside the allowed window, the system will not reset and runs normally.

3. Final Answer:

   The system will run normally without reset -> Option C

4. Quick Check:

   Refresh at 50ms inside 40-60ms window = no reset [OK]

Hint: Check if refresh delay fits inside allowed window [OK]

Common Mistakes:

• Assuming 50ms is too early or too late
• Confusing delay units
• Ignoring the watchdog window limits

4. The following code causes the system to reset unexpectedly. What is the likely error?

void main() {
  WWDG_Init();
  while(1) {
    WWDG_Refresh();
    delay(30);  // delay in ms
  }
}

Assuming the allowed refresh window is 40ms to 60ms.

medium

A. Not calling WWDG_Refresh() frequently enough

B. Delay is too long causing late refresh

C. Missing WWDG_Init() call

D. Refreshing the watchdog too early before the window opens

Solution

1. Step 1: Check refresh timing relative to window

   The code refreshes the watchdog immediately, then delays 30ms, so refresh happens every ~30ms.

2. Step 2: Compare with allowed window

   Since the window starts at 40ms, refreshing at 30ms is too early and causes reset.

3. Final Answer:

   Refreshing the watchdog too early before the window opens -> Option D

4. Quick Check:

   Refresh before 40ms window causes reset [OK]

Hint: Refresh only after window opens, not before [OK]

Common Mistakes:

• Thinking delay is too long
• Ignoring refresh order
• Assuming missing init causes reset here

5. You want to design a system using a window watchdog timer that resets if the refresh happens outside 100ms to 200ms window. Which approach correctly implements this behavior in embedded C?

hard

A. Refresh the watchdog every 150ms

B. Refresh the watchdog every 50ms

C. Refresh the watchdog every 250ms

D. Refresh the watchdog only once at startup

Solution

1. Step 1: Understand the allowed refresh window

   The watchdog must be refreshed between 100ms and 200ms to avoid reset.

2. Step 2: Evaluate each refresh timing option

   Refreshing at 50ms is too early, 150ms is inside window, 250ms is too late, and once at startup is insufficient.

3. Final Answer:

   Refresh the watchdog every 150ms -> Option A

4. Quick Check:

   Refresh inside 100-200ms window avoids reset [OK]

Hint: Refresh watchdog only inside allowed time window [OK]

Common Mistakes:

• Refreshing too early or too late
• Refreshing only once
• Ignoring window timing constraints